PROJECT SUMMARY/ABSRACT Myelofibrosis (MF) is a chronic, ultimately fatal hematologic malignancy characterized by progressive fibrosis of bone marrow, leading to severe anemia, hepatosplenomegaly, and debilitating constitutional symptoms with cachexia. Treatment options remain extremely limited because only one FDA-approved drug currently exists for MF. This drug may reduce splenomegaly and constitutional symptoms but only minimally reduces fibrosis or abundance of malignant HSCs, the primary drivers of disease. The inability to reverse fibrosis and the malignant clone is a major reason for continued poor prognosis in MF with ?40% five-year survival. Oncologists currently rely on bone marrow biopsy and spleen size measured by physical examination or anatomic MRI to assess disease status and response to therapy in MF. Although regarded as the gold standard for analyzing bone marrow, biopsy has several fundamental limitations as a test for status of a disease known to have extensive heterogeneity in different anatomic sites of hematopoietic marrow. Biopsy samples only a small volume of bone marrow from a single site, the iliac crest. In patients with extensive fibrosis in bone marrow, biopsy frequently recovers no tissue (?dry tap?), leaving patients and physicians with no information about bone marrow composition and severity of disease. As an invasive, painful procedure, patients only tolerate a limited number of bone marrow biopsies. Measurements of spleen volume are non-invasive and easy to perform but fail to address the fundamental cause and site of pathology, progressive fibrosis in bone marrow. To advance pre- clinical studies in pathophysiology of MF, drug development, and ultimately clinical oncology, we will investigate quantitative bone marrow MRI as a biomarker for disease status and response to therapy. We will assess bone marrow composition and architecture using clinically-approved MRI sequences for cellularity (fat/water, Dixon method), diffusion of water (DWI), and macromolecular structure (magnetization transfer saturation, MTS). We will analyze imaging data by parametric response mapping (PRM), which captures spatial and temporal changes in imaging data from the same patient over multiple studies. PRM markedly improves detection of early effects of therapy and predicts long-term outcome in patients with multiple types of malignancies. To advance bone marrow MRI as an imaging biomarker in MF, we will accomplish the following aims: 1) Validate quantitative MRI metrics for bone marrow in mouse models of MF; 2) Quantify response to established and investigational therapies in mice with genetic driver mutations mirroring patients; and 3) Conduct a prospective initial clinical trial using quantitative MRI to monitor response to therapy in MF. We expect this research to show that quantitative bone marrow MRI detects response to therapy in MF, allowing non-invasive measurements of disease heterogeneity and assessment of drugs to reverse bone marrow fibrosis. Relevance: The ability to track heterogeneity of disease throughout the skeleton by imaging represents a transformative advance over bone marrow biopsy that ultimately will improve quality of life and care for patients with MF.